A "flip chip" is a semiconductor die having terminations all on one side in the form of solder pads or bump contacts. After the chip has been passivated or otherwise treated, it is flipped over for attaching to a matching substrate. A "BGA" or "Ball Grid Array" chip is similar to the "flip chip", except that the chip is mounted on a carrier, with wires connecting the chip to the carrier, and the carrier has the solder pads for connection to the substrate.
The use of flip chips and BGA chips have inherent advantage over other methods in high-density electronic packaging because it provides an area array (2 dimensional) that interconnects the chip and the substrate (Tummala and Rymaszewski, 1989). However, due to the high production cost of today's technology, its market share among semiconductor chip industry is low as yet. As the flip-chip and BGA technology improves, their uses are expected to increase drastically in the near future. The Semiconductor Industry Association projects a 250-fold increase in IC functional density over the next 15 years. Flip-chip technology and BGA technology is expected to play a major role in accomplishing such a goal due to its capability of connecting high numbers of i/o and compactness. However, the predicted growth rate by experts all depends on the ability to reduce production cost significantly.
Traditionally, flip-chip technology uses ceramic material as chip carrier or substrate (see Miller, L. F., 1969, "Controlled Collapse Reflow Chip Joining", IBM J of Res. and Dev., v. 1 3, 239-250). Because of the similarity of the thermal-expansion coefficients of the chip and ceramic, thermal stresses during thermal excursions were not a serious problem.
Recently, the desire for low cost mass production has resulted in the growing use of organic materials for substrates (Flip chip on board or "FCOB"). The current process typically uses solder to connect the chip to the board. Although efforts are being made to use electrically conductive polymers to connect the chip to the board (see Dion, J., Borgesen, P., Yost, B., Lilienfeld, D. A. and Li, C. Y., 1994, "Material and Reliability Considerations for Anisotropically Conductive Adhesive Based Interconnects", Mat. Res. Soc. Symp., Proc. 323, Pittsburgh, Pa.), such development is still in the research stage.
In FCOB, the difference in thermal-expansion coefficient between the chip (2.5 ppm/.degree.C.) and the organic substrate (15 ppm/.degree.C.) causes significant shear strain on the interconnects during temperature cycling, ultimately resulting in fatigue cracking and electrical failure (Machuga, S. C., Lindsey, S. E., Moore, K. D. and Skipor, A. F., 1992, "Encapsulation of Flip Chip Structures", IEEE/CHMT Symposium, p.53). This is particularly true as the size of the chip increases because the thermal stress in the interconnect increases with DNP (distance from the neutral point).
This problem can be significantly reduced by encapsulating the space between the solder joints with encapsulant to provide mechanical reinforcement. Because the encapsulation is provided mainly under the chips, this process is called "underfill encapsulation". Underfill encapsulation is different from encapsulation of other packages such as DIP or PGA, which require encapsulation on all sides of the chip.
Underfill encapsulation has resulted in more than a ten-fold increase in the reliability of the flip chip on board technology (Nakano, F., Soga, T., Amagi, S., 1987, "Resin Insertion Effect on Thermal Cycle Resistivity of Flip-Chip Mounted LSI Device", ISHM Conf., 536-541; Suryanarayana, D., Hsiao, R., Gall, T. P., McCreary, J. M., 1991, "Enhancement of Flip Chip Fatigue Life by Encapsulation", IEEE CHMP, v. 14, 218-223). The same is true for BGA technology.
Currently, most flip chip and BGA packages are encapsulated by dispensing the encapsulant along the periphery of one or two sides of the chip. Capillary action (i.e. a surface-tension phenomenon) drives the encapsulant through the space between the chip and the board. After the filling is complete, the board is taken to an oven where it is cured. The current encapsulation process has the following problems:
(1) Filling and curing: Because the current process fills the cavity (space between the chip and the board) by capillary action, it is very slow and could be incomplete, resulting in voids. The filling problem becomes even more serious as the chip size increases. The fill time is proportional to the square of the length of the chip. (See Suryanarayana, D., Wu, T. Y., Varcoe, J. A., 1993, "Encapsulaits Used in Flip-Chip Package", 43rd ECTC, Orlando, Fla.) For example, in a typical encapsulation operation, the filling takes about 2 minutes for a small chip (1/4") to 15 min. for a large chip (3/4"). PA1 (2) Adhesion: Adhesion of the fluid to the chip is essential for achieving good reliability of the encapsulated chip. High wettability of the fluid requires low surface tension which, in turn, leads to an even slower cavity filling. PA1 (3) Encapsulant: Development of materials suitable for flip chip encapsulation is difficult. The encapsulant needs to have good fluidity, wettability, matching thermal expansion coefficient with the solder and with appropriate curing kinetics. It is difficult to satisfy all these at the same time. Currently, most encapsulants are epoxy-based thermoset which are filled with solid fillers (O'Malley, G., Giesler, J. and Machuga, 1994, "The Importance of Material Selection for Flip Chip on Board Assemblies", IEEE CPMT, Pt. B, v. 1 7, 248). On one hand, It is desirable to increase the filler content to match the thermal-expansion coefficient of the solder but on the other hand, the viscosity increases sharply with the filler content, which make the flow more difficult. Another problem with the current encapsulant is because of its thermosetting nature, the repair of the board is difficult. PA1 1. Banerji's method depends first on a partial vacuum to "suck out" the air from the under-chip area and let it "bubbling through the polymer infiltration" (line 54, p. 2)--a method inherently impossible to get all the air out. This is even after the air is let in (at the atmospheric pressure) again to push the polymer to infiltrate the partially-vacuumed under-chip area. The air in the chamber always maintains a uniform pressure (i.e. no gradient) except at the instant when the vacuum is applied or removed. On the contrary, our invention pushes the air out of the under-chip area by the flow front of the liquid polymer coming in at one point (gate) driven by a pressure gradient from the injection system. The flow front advancement can be predicted by computer simulation to ensure that all the air is evacuated (not bubbling through). PA1 2. Because of high pressure gradient (which can be as high as 1000 psi), the filling time in the present invention is less than one second, not "about 50 seconds" in Banerji's patent, or 5 to 20 minutes in conventional underfill method. PA1 3. The fillet geometry is considered critical to the reliability of the package. In Banerji's invention, the shape of the fillet is determined by the force balance of the surface tension and gravity on the encapsulant. This limits controllability of the shape of the fillet, because neither factor can be controlled. In contrast, in the present invention, the fillet shape can be controlled by the proper design of the mold. Especially when in-mold curing is used, the fillet shape will exactly follow that of the mold cavity. PA1 4. Also, because of the fast filling time with the current invention, fast-curing encapsulant can be used to reduce the cycle time. U.S. Pat. No. 5,381,599, granted to Hall, G. L., in January 1995, for "Liquid Crystal Polymer Encapsulated Electronic Devices and Methods of Making the Same", uses a high pressure and high temperature process to encapsulate flip-chips with a rigid liquid crystal polymer. The encapsulation is applied over the flip-chips. Epoxy polymer can be applied over and part of under the chip before applying liquid crystal polymer. This process cannot fill the cavity underneath the chip completely, which will be detrimental to the reliability of the package. Also, the encapsulant used is liquid crystal polymer which is as yet less popular than epoxy. PA1 (1) The filling time can be reduced dramatically compared to dispensing encapsulation process. For example, for a 0.3" chip, the fill time can be as much as 100 times or more smaller. For the test case described below, the filling time was reduced by 100 times. The curing process can also be shortened by using a faster-curing encapsulant, which is impractical in the current process. PA1 (2) The filling process can be done at room temperature if in-mold curing is not needed, as compared to the prior-art dispensing process which typically requires heating. PA1 (3) Another benefit that can be achieved from this process is a potential increase in reliability of the package. First, as indicated in Giesler et al. (Giesler, J., O'Malley, G., Williams, M. and Machuga, S., 1994, "Flip Chip on Board Connection Technology: Process Characterization and Reliability", IEEE CPMT, pt. B, v. 1 7, 256-263), reliability of the package increases as the encapsulation temperature decreases. Because in the new process, encapsulation can be conducted at room temperature, the reliability of the package increases compared to other processes which are done at elevated temperature. Second, it is well known (see Giesler et al.) that the reliability of the package depends on the fillet shape. The current process can control the fillet shape better than the dispensing process by designing the mold appropriately. Third, for the in-mold curing process, the curing will take place under high pressure which will enhance the adhesion of encapsulant to the package. The enhanced adhesion will improve the reliability of the package. PA1 (4) The selection or development of encapsulant will become much more flexible. For example, it is no longer necessary to compromise the wettability and the surface tension. Also, the process will be able to handle materials with higher viscosity thus allowing more fillers to be added to the encapsulant in order to better match the thermal-expansion coefficients of the encapsulant and the solder. It can also handle thermoset material with much faster curing kinetics. It will also be possible to add rubbery plastic to the encapsulant. This will soften the cured encapsulant when heated, making the reworkability of the package possible.
Because the filling is slow, the encapsulant has to be a slowly curing material. Thus, curing of the encapsulant typically requires several hours in an oven. The slow filling and curing process is detrimental to mass production.
Some recent inventions have tried to improve the flip-chip encapsulation process.
U.S. Pat. No. 5,218,234 granted to Thompson, K. R., Banerji, K. and Alves, F. D., in 1993 for a "Semiconductor Device with Controlled Spread Polymeric Underfill", teaches creating a window-frame shaped opening in the film on the substrate. The semiconductor device lies in the interior perimeter of the opening. The window frame opening in the film serves to confine the encapsulant which fills the space between semiconductor device and substrate to within the opening. This prevents unwanted spread of the encapsulant and help form an ideal fillet geometry. Other than this, this invention still suffers from the disadvantages that the dispensing process have.
U.S. Pat. No. 5,203,076, granted to Banerji, K., Alves, F. D., and Darveaux, R. F., in 1993 "Vacuum Infiltration of Underfill Material for Flip-Chip Devices", applies and removes vacuum to encapsulate the chips. In this process, a bead of underfdill material is provided on the substrate about the periphery of integrated circuit. Vacuum is then applied to evacuate the area between integrated circuits and the substrate through the underfill encapsulant. After that, the vacuum is removed forcing the underfill material into the evacuated area. This process, however, involves three steps and requires a vacuuming process which is not effective for highly viscous encapsulant. The key differences between Banerji's patent and our invention can be summarized as follows: